Air Conditioning - Extending Ducts

I need to extend 3 ducts into a new 2 story addition.

The portion of the room that we added onto on the upper floor had 3 registers, each connected to a separate duct. They were situated below the windows on 3 outside walls. The room below (also added onto) had no registers but the ductwork for the upper level passed through the overhead floor joists.

One of the registers on the top floor needs to either be removed or extended because it is now in the center of the room. My plan is to simply flip that one so it points toward the lower level instead of the upper level. I want to extend the remaining two ducts so that the original registers still blow air but there are two more registers on the far end of the new addition. So, basically, I want to 'T' the ducts so they feed both the original registers and the new ones.

First, is there anything wrong with having multiple registers on the same line?

If not, what type of fittings and boots should I use to accomplish the "T" to the original registers? I tried a typical 6" T with a standard 6" register boot but it took up so much room in the joist cavity below that there wouldn't be enough clearance to drywall the ceiling. I need something with a low profile but I haven't seen anything in Lowes/Home Depot that looks like it would work.

Here are some other details:

total ducts to be extended: 3
duct size: 6"
total registers in area before: 3
total registers in area after addition : 5 (4 upstairs, 1 downstairs)
sq ft of house before addition: 2700
sq ft after addition: 3200
system type: Train Heat Pump

My primary concern is cooling since both floors have gas fireplaces. The downstairs level is actually a walk-out basement but the new section is well above grade. It's always been very cool down there and the new space is a theater room with no windows. ...so I only see the need to add one register in that area.

On the upstairs level we have lots of windows so I'm worried about the temp in the summer.

Both sections are 430 sq ft.

Our heat pump is 12 years old so it will likely need replaced in the next few years. Therefore, I'm not terribly concerned about the sizing of the system but I do plan to cut a new return duct for make up air. Can I do this on the basement level right where it comes off the heat pump unit?

Sorry, I realize this is a lot of info. Thanks!

You need to back up! You need to do a heat load on the new wing. This will tell you how much air you need. Id be willing to bet money your old unit will not be able to handle the extra load.

Here are the results from heatload.com.

Not sure how it will look here but I'll paste it in.

I do expect to have to replace the heat pump within the next year or two. I just didn't want to dump that on top of everything else right now considering that we make heavy use of propane during the winter. (2 fireplaces and 2 wall heaters for a total of 110k btu).

My primary concern at the moment is getting the ducts extended correctly so they're located correctly and there arent issues when the new system is installed. Still, I'm open to any advice since I'm certainly no expert.

I neglected to state the heat pump model before because I was away from home at the time. It's actually a Trane TWE048P130BO.

This info is for the 2 rooms that were expanded by the addition. This includes both existing square footage and what we added. It didn't really seem to make sense to only include what we added since the calculations ask for things like exposed walls, windows and other construction characteristics and we did some things like adding a new roof to the whole section plus some additional windows and doors in the old part. In reality, we only added 254 sq ft to each floor but I used 432 sq ft instead since that's the size of each of the two rooms being served by the system. Both rooms open to the main house structure on one wall so there are 3 exposed walls on both levels.

thanks again.




First Floor Specifications
Room Name Heat Loss Heat Gain
Other
6399 btu
2329 btu
TOTAL FLOOR
RESULTS 6399 btu 2329 btu


Second Floor Specifications
Room Name Heat Loss Heat Gain
Family Room
13736 btu
9078 btu
TOTAL FLOOR
RESULTS 13736 btu 9078 btu


House Specifications
TOTAL HEAT LOSS 20135 btu
TOTAL HEAT GAIN 11407 btu

well, I need to get on with drywalling the first floor ceiling so I've decided to extend only one duct and leave the other one as is. ...will still point one downstairs to the theater room for AC.

I'm going to run a "spare" 6 inch duct all the way back to the plenum and just leave it unattached in case we ever need it. That way, when I replace the heat pump, I can let the pros make the call on whether or not to hook it up and they can worry about balancing the system correctly.

This will give me 3 6" ducts into the 18X24' room on the second floor and one 6" duct into the windowless theater room below.

Any comments on this plan of attack?